Cytochrome P450s represent a superfamily of enzymes, most of which metabolize foreign compounds. Drugs are usually inactivated and converted to derivatives that can easily be eliminated from the body. Chemical carcinogens are metabolically activated by P450s. A large degree of species differences in P450 catalytic activities have been noted indicating that human P450s must be directly studied to understand risk to drug and carcinogen exposure. To examine human P450 catalytic activities, cDNA expression was performed using several systems. Vaccinia virus expressed P450s were analyzed for their activities toward nicotine. CYP2B6 exhibited the highest activity toward nicotine C oxidation followed by CYP2C9 and CYP2E1. Oxidation of nicotine proceeds with the production of an activated iminium ion that is capable of covalently binding to cellular macromolecules. Thus, this might account, in part, for tobacco smoke associated toxicity. Human P450s have also been produced using B-lymphoblastoid cells to study mutagen activation with a whole cell system. Baculovirus has been used to produce large amounts of P450 enzymes that are examined for catalytic activities. The human NADPH P450 oxidoreductase was expressed in baculovirus and shown to be involved in activation of mitomycin C. A number of human P450 genes have been isolated and are being sequenced to search for dinucleotide and trinucleotide repeats that can be used in linkage analysis to determine association of variant or mutant P450s with environmentally based disease.